Activator compositions for cyanocrylate adhesives

ABSTRACT

An activator composition for accelerating hardening and achieving cure-through-volume of cyanoacrylate adhesives) comprising; (a) one or more compounds selected from the group consisting of :(i) pyrazine; or a pyrazine derivative; said pyrazine derivative being pyrazine substituted with at least one electron-releasing group; (ii) pyridine N oxides substituted with at least one electron-releasing group; or (iii) 2,6 pyridines being pyridines substituted in the 2- and 6-positions by substituents, at least one of the substituents being electron-releasing provided that both substituents are not methyl; and (b) a volatile solvent carrier for the compound.

FIELD OF THE INVENTION

This invention relates to activator compositions, particularly wellsuited for accelerating the hardening of cyanoacrylate adhesives. Theinvention further relates to a process for the accelerated bonding ofsubstrates using cyanoacrylate adhesives.

BRIEF DESCRIPTION OF RELATED TECHNOLOGY

Adhesive compositions based upon cyanoacrylate esters are well known andhave found extensive use, because of their rapid cure speed, excellentlong-term bond strength, and applicability to a wide variety ofsubstrates. They generally harden after only a few seconds, after whichthe joined parts exhibit at least a certain degree of initial strength.

If the cyanoacrylate (CA) adhesive is conventionally applied in arelatively thick layer in the joint gap or relatively large amounts ofadhesive are applied so that relatively large drops of adhesive protrudefrom between the parts to be joined, rapid hardening throughout theadhesive may rarely be achieved, i.e. cure-through-gap orcure-through-volume (CTV) performance may be unsatisfactory.

With certain substrates, particularly substrates having acidic surfaces,such as wood or paper, the polymerisation reaction may be retarded,often times to an unmanageable extent. Moreover, unless the adhesive isgelled or rendered thixotropic by appropriate additives to confer suchproperties, the wood or paper substrates, due to their porosity, tend todraw the adhesive out of the joint gap by capillary action beforehardening has taken place in the gap.

Heretofore efforts have been made to accelerate the polymerisation ofsuch CA adhesives by means of certain additives. Addition ofaccelerators directly to the adhesive formulation is possible to only avery limited extent, however, since substances having a basic ornucleophilic action, which would normally bring about a pronouncedacceleration of the polymerisation of the cyanoacrylate adhesive, aregenerally used, it is usually at the expense of the storage stability ofsuch formulations.

Addition of such accelerators shortly before application of the adhesiveresults in virtually a two-component (two-part) system—in other wordsthe adhesive and the accelerator are stored apart—the accelerator wouldnot normally be mixed with the adhesive for storage purposes. However,such method has the disadvantage that the working life is limited afterthe activator has been mixed in. In addition, with the small amounts ofactivator that are required, the necessary accuracy of metering andhomogeneity of mixing are difficult to achieve. Moreover, use of such atwo-part system is often seen as cumbersome to the end user, andsometimes only modestly improves the intended result.

Activators are also used in the form of a dilute solution which iseither applied beforehand onto a substrate such as a part which is to bebonded, and/or is applied onto the adhesive where it is still liquid(before the adhesive has fully cured) after the substrates have beenjoined. The solvents used for such dilute solutions of activators aregenerally low-boiling organic solvents, so that they may be readilyevaporated, leaving the activator on the substrate and/or on theadhesive.

Japanese Patent Application Nos. JP-A-62 022 877, JP-A-03 207 778 andJP-59-66471 propose the use of solutions of various amines in solutionas accelerators for cyanoacrylate adhesives. U.S. Pat. No. 3,260,637 ofvon Bramer discloses the use of a range of organic amines (excludingprimary amines) as accelerators for cyanoacrylate adhesives,particularly for use on metallic and non-metallic substrates.

N,N-dimethyl-p-toluidine has been widely used as an accelerator for theaccelerated hardening of cyanoacrylate adhesives. This chemical providesa short duration of the surface activation, which does not permit longwaiting times between application of the accelerator solution beforehandto the substrates to be bonded and the subsequent bonding process. Inaddition, the use of N,N-dimethyl-p-toluidine in some countriesoftentimes involves rigorous regulatory labelling requirements.

Basicity of an accelerator substance is not a sufficient criterion foridentifying solutions which are acceptable in practice in terms ofapplication technology. Many substances, such as alkylamines,1,2-di-(4-pyridyl-ethane), 4,4′-dipyridyl disulfide,3-(3-hydroxypropyl)pyridine, 1,2-bis(diphenylphosphino)-ethane,pyridazine, methylpyridazine or 4,4′-dipyridyl, are so basic ornucleophilic that spontaneous superficial hardening takes place at theadhesive interface (shock hardening) before the activator is able toinitiate polymerisation throughout the liquid adhesive (usually a dropor layer) by dispersion processes such as by convection and diffusion.The result is that an often cloudy polymerisation occurs at the surfaceonly. With other compounds, such as oxazoles, the basicity is evidentlytoo low, and the hardening is often too slow for practical purposes.

German Patent DE-A-22 61 261 proposes accelerator substances containingthe structural element —N═C—S—, including 2,4-dimethylthiazole. U.S.Pat. Nos. 5,567,266 (Liu) and U.S. Pat. No. 5,643,982 (Liu) describeaccelerator compositions comprising amine compounds. EP 0 822 242 andU.S. Pat. No. 5,749,956 (Fisher et al.) describe non-ozone depletingnon-flammable co-solvent useful for carrying inter alia accelerators.

European Patent Publication No. 0 271 675 A2 of Three Bond Co. Ltd.describes a primer for CA adhesive for use in bonding non-polar orhighly crystallized resins such as polyolefins,polyethyleneterephthalates, nylons, fluorine-containing resins, soft PVCfilms and the like which are notorious as being ordinarily difficult tobond. The primer comprises (A) a compound selected from the groupconsisting of benzene ring compounds having aldehyde groups and nitrogenor oxygen atom-containing heterocyclic compounds having aldehyde groups(such as 2-pyridine carboxylaldehyde, 2,6-pyridine carboxylaldehyde andpyrrole 2-carboxylaldehyde), and (B) an organic amine compound. The EP'675 publication states that in bonding non-polar or highly crystallizedresins using a CA adhesive the primer instantaneously exhibits a highbonding strength at ambient temperature by simply applying such primeronto a surface of one of the resins, applying the CA adhesive onto asurface of the other resin and bringing both surfaces into contact witheach other, thus achieving “zero gap” and an instantaneous high bondingstrength.

British Patent Specification No. 1 230 560 of International ChemicalCompany Limited (ICC) describes CA adhesive compositions containingcertain substituted heterocyclic compounds as accelerators. Thecompositions may be presented in a two-part form, the first partcomprising the CA adhesive and the second part comprising at least oneof the substituted heterocyclic compounds, preferably in solution in anorganic solvent. In the compositions in which the heterocyclic compoundis an iminoethylene-substituted triazine or pyrimido-pyrimidine, theheterocyclic compound is invariably present in one part of a two-partcomposition because iminoethylene-substituted triazines andpyrimido-pyrimidines accelerate the polymerisation so rapidly they mustbe kept apart from the CA composition before use.

Japanese Patent Abstract Publication No. 62018485 of Alpha Giken KK alsodescribes a primer for a CA adhesive and is not concerned with anactivator for good CTV performance.

Activator solutions are often applied by spraying. There is a demand foractivator solutions which can be applied in excess volumes (e.g. asdrops) onto an adhesive already present on a substrate (e.g. in the formof a bead or fillet).

Aliphatic hydrocarbons such as heptane are often used as a carrier forCA activators.

Notwithstanding the state-of-the-art, it would be desirable to providenew activators and combinations thereof with different physicalproperties from the activator substances than those known heretofore. Inaddition, it would be desirable for such activators to have a pronouncedaccelerating action on the cyanoacrylate adhesive and low volatility.Such physical properties will allow for the application of theactivators either onto the substrate to be bonded prior to applicationof the cyanoacrylate adhesive or after application of the cyanoacrylateadhesive onto the substrate to be bonded as an overspray. It would alsobe desirable to find chemicals useful as activators for cyanoacrylateadhesives which are subject to regulatory labelling requirements lessrigorous than those currently in place for N,N-dimethyl-p-toluidine.

SUMMARY OF THE INVENTION

According to one aspect, the present invention provides a composition(in particular suitable for use as an accelerator composition for CTV ofcyanoacrylate adhesives) comprising:

-   -   (a) one or more compounds selected from the group consisting of:        -   (i) pyrazine; or a pyrazine derivative;            -   said pyrazine derivative being pyrazine substituted with                at least one electron-releasing group;        -   (ii) pyridine N oxides substituted with at least one            electron-releasing group; or        -   (iii) 2,6 pyridines being pyridines substituted in the 2-and            6-positions by substituents, at least one of the            substituents being electron-releasing provided that both            substituents are not methyl; and    -   (b) a volatile solvent carrier for the compound.

In the groups (i) to (iii) above (the groups (i) to (iii) are used forconvenience and mixtures of compounds within a designated group and/orbetween groups is encompassed by the use of the phrase “at least one”)the electron releasing effect of the substituent is measured relative tohydrogen—in other words a substituent which is considered moreelectron-releasing than a hydrogen substituent is desired. Usually theelectron-releasing effect would be manifested in a net increase in thebase strength of the substituted compound compared to the correspondingSubstituted compound. Such a net increase in the base strength of thesubstituted compound compared to the corresponding unsubstitutedcompound is desirable for all compounds of groups (i) to (iii) above.

This base strength effect is particularly desirable for the substitutedpyrazines of group (i) and the pyridine N-oxides of group (ii).

A net increase in the base strength of the substituted compound comparedto the corresponding unsubstituted compound is seen as desirable alsofor the 2,6 pyridines of group (iii). In the group (iii) compounds theproximity of the substituents of these compounds to the N atom of theheterocycle also has an effect on the nucleophilicity of the (N atom) ofthe compounds which is thought to occur due to steric hindrance effects.In this respect it is desirable that the compounds of group (iii) arenot substituted in both the 2-and 6-position by methyl. The presentinventors have found that such substitution (for example inlutidine-2,6-dimethyl pyridine) results in a compound which is tooreactive toward cyanoacrylate (both substituents beingelectron-releasing and together not providing sufficient sterichindrance) and does not achieve the desirable effects set out above.Accordingly at least one of the substituents in the 2-or 6-positionsshould not be methyl and desirably at least one of those substituents ismore sterically hindering toward the N atom of the heterocycle than amethyl group. It may be that both substituents are each more stericallyhindering than a methyl group would be in the 2-and 6-positions.

The compounds of groups (i) to (iii) may be additionally substitutedwith one or more other groups such as one or more electron-withdrawingand/or electron-releasing groups and/or groups that are neitherelectron-withdrawing nor electron-releasing. Overall it is desirablethat the net effect of all substituents is to increase the base strengthof the substituted compound as compared to the unsubstituted one.

In the case of pyrazines where there are two N heteroatoms it isdesirable that each N heteroatom is sterically hindered each desirablyby two N heteroatom adjacent groups particularly where there are twoelectron-releasing groups. Desirably both groups are electron-releasinggroups. In this respect N heteroatom adjacent position is used to meanthe substituent is on a carbon atom of the heterocycle which is directlybonded to a N heteroatom of the heterocycle.

In the case of pyridine, the substituent(s) is (are) desirablysufficiently bulky to decrease the nucleophilicity of the Nheteroatom(s) of the heterocycle. In other words the substituents shouldbe such as to make the compound less reactive (as accelerator) toward acyanoacrylate adhesive. Less reactive is used herein in the comparativesense against the parent pyridine compound.

Preferably the electron releasing substituent(s) are selected from oneor more of straight chain, branched chain, or cyclic (each optionallysubstituted) alkyl groups particularly where there are twoelectron-releasing . Typically up to 40 carbon atoms may be present inthe substituent though this upper limit is not seen as critical in atleast as so far as electron-releasing effects are to be considered.Typical substituents such as methyl, ethyl, propyl (in particularn-propyl or iso propyl), butyl (in particular n-butyl, iso butyl, tertbutyl), pentyl such as n-pentyl or cyclopentyl, or hexyl such ascyclohexyl may be employed.

These chains may themselves be further substituted (provided that thesubstitution results in an electron-releasing substituent). Theadditional substituent(s) may be one or more substituents selected fromthe group consisting of:

-   -   halo, CR═CR¹R², CF₃, CCl₃, OC(O)R, COOR, COR, OR, SR, CONR¹R²,        NO₂, NO₃, SOR, SO₂NR₂, NRSO₂R, SO₂R³, SO₃R³, PO(OR³)₂ and        optionally substituted C₆-C₂₀ aryl, , or aryloxy, CSOR³, COONR³        ₂, NRCOOR, NRCOR, N═N—R³, OOR³, SSR³, OC(O)OR³, N(OR³)₂,        P(OR³)₂, SOR³, OSR³, wherein R, R¹ and R² (which may be the same        or different) are H, optionally substituted C₁-C₁₀ alkyl, or        optionally substituted C₆-C₂₀ aryl, and R³ is optionally        substituted C₁-C₁₀ alkyl, or optionally substituted C₆-C₂₀ aryl.        Halo includes chloro, bromo, fluoro and iodo.        Pseudohalo-radicals such as CN, SCN, OCN, NCO, NCS are also        included within the group of useful substituents.

Some electron-withdrawing groups which can be employed for furthersubstitution directly on the heterocycle include hydroxy, methoxy,methanol, thio and thiol.

Specific examples of compounds of groups (i) to (iii) include compoundsutilised in the worked examples below.

Mixtures of activators can be used both from within those of the presentinvention and with those of the co-pending International application no.PCT/IE 01/00063 (filed on 11 May 2001 with the applicant Loctite (R&D)Limited) the entire contents of which are incorporated herein byreference. For example an intelligent selection of activators would beto combine an activator with good pre-spray properties (such as 4-methylpyridine-N-oxide from the present invention or from 2,2′ dipyridyldisulphide or 5-nitroquinoline (from PCT/IE 01/00063) with someactivators (for example the pyrazines that have good post sprayproperties.

Specific examples include 6-methyl-2-pyridine methanol or 2,3-dimethylor 2,5 dimethyl or 2-methyl-3-n-propyl or 2 methoxy 3 methyl substitutedpyrazines separately mixed with 2,2′ dipyridyl disulphide or5-nitroquinoline (from PCT/IE 01/00063) or mixed with 4-methyl pyridinen-oxide described herein. Also within the ambit of the present inventionare mixtures containing more than one activator with good post sprayproperties with more than one activator with good pre spray properties.

The compositions of the present invention may additionally comprise acomponent:

-   -   (c) an organic compound containing the structural element,        —N═C—S—S—.

According to a special feature, the present invention provides anactivator composition for the accelerated hardening of cyanoacrylateadhesives, wherein the activator comprises a mixture of an aromaticheterocyclic compound according to the composition of the inventiondescribed above and an organic compound having the structural element,—N═C—S—S—.

According to a further feature, the present invention provides anactivator composition for the accelerated hardening of cyanoacrylateadhesives, wherein the activator comprises a mixture of a compositioncomprising components (a), (b) and (c) above where the compound of group(c) is an organic compound having the structural element, —N═C—S—S—,more particularly the structural element, —N═C—S—S—C═N—, more especially—N═C—S—S—C═N— wherein the N═C and C═N double bonds are parts of aromaticheterocyclic rings.

The activator compositions of the present invention are for theaccelerated hardening of a cyanoacrylate adhesive throughout theadhesive. The compositions of the invention are particularly directed togood CTV performance, in particular accelerated hardening throughout theadhesive, in drops of adhesive or relatively large layers of adhesive ina joint gap. A joint gap is a gap between two surfaces (often eachsurface is on a separate substrate on a separate substrate thoughsurfaces on the same substrate are also of interest) which are spacedapart from each other. In this respect the term “spaced apart” is usedto define a relationship between substrates which defines the gapbetween surfaces as larger than the “zero gap” achieved when twosubstrate surfaces are in contact. The term zero gap is often employedto refer to the direct contact between surfaces allowing forirregularities/imperfections on the surface which mean that perfectcontact is not achieved. In general (and in particular for successfulCTV through a bulk of cyanoacrylate) a gap having a greater width than10 microns is of interest. The depth of the adhesive drop or layerperpendicular to the substrate surface is suitably in the range 0.5 mm-2 mm, particularly 0.75 mm -1.25 mm.

In the said organic compound containing the structural element—N═C—S—S—, the N═C double bond may optionally be part of an aromaticsystem, which may suitably be monocyclic, bicyclic or tricyclic. Forexample, the N═C double bond may suitably be part of an aromaticheterocyclic ring having one or more N hetero atoms in the ring,optionally with one or more other hetero atoms selected from S and O.The heterocyclic ring may be substituted.

Desirably the said organic compound contains the structural element—N═C—S—S—C═N—, in which case both the N═C double bond and the C═N doublebond may optionally be part of aromatic systems as described above,suitably two similar aromatic systems. More desirably the said organiccompound is selected from dibenzothiazyl disulfide,6,6′-dithiodinicotinic acid, 2,2′-dipyridyl disulfide, andbis(4-t-butyl-1-isopropyl-2-imidazolyl) disulfide. Of course,combinations of these organic compounds may also be used. Organiccompounds having structural element —N═C—S—S—, which are useful asaccelerators for accelerating the curing of CA adhesives if diluted in asolution, are described in WO 00/39229 and the corresponding U.S. Patentof Henkel KGaA, the entire contents of both of which are incorporatedherein by reference.

In a further aspect the present invention also relates to the use of atleast one aromatic heterocyclic compound having at least one N heteroatom in the ring(s) and substituted on the ring(s) with at least oneelectron-releasing group in the manufacture of a cure through volumeaccelerator composition for cyanoacrylate adhesives.

In this further aspect said at least one aromatic heterocyclic compoundmay be a member selected from the group consisting of pyridines,quinolines, pyrimidines and pyrazines substituted on the heterocyclering or at least one of the heterocycle rings with at least oneelectron-releasing group.

An aromatic heterocyclic compound may suitably be monocyclic, bicyclicor tricyclic. The N hetero atoms(s) may be present in one or more of therings. Two or more heterocyclic rings may be fused, or a heterocyclicring may be fused to one or more carbocyclic rings. A heterocyclic ringmay suitably be a 5-or 6-membered ring and may suitably have one or twoN-atoms in the ring. A 6-membered heterocyclic ring is particularlysuitable. In the case of two fused heterocyclic rings, the total numberof N-atoms is suitably not more than three. The aromatic heterocycliccompounds are suitably substituted on the ring carbon atoms. Acarbocyclic ring fused to a heterocyclic ring may suitably have 6 carbonatoms and/or may be an aromatic ring. A compound comprising aheterocyclic ring fused to a carbocyclic ring may be substituted byelectron-withdrawing group(s) on either or both of the heterocyclic andcarbocyclic rings.

In the various aspects of the invention described above the number ofelectron-releasing group(s) (which may be the same or different) may befrom 1 to 3 groups on the ring or per ring, for example 1 or 2 groups onthe ring or per ring.

The criterion that the electron-releasing group increases the basestrength of the substituted compound compared to the correspondingunsubstituted compound may be determined by pKa measurement in waterunder standard conditions (e.g. 25° C. and zero ionic strength) byconventional means or using a software package which calculates pKa forthe reaction of the positively charged protonated base BH⁺ with water asindicated in the reaction such as “ACD/pKa Calculator” available fromAdvanced Chemistry Development, 133 Richmond Street West, Suite 605,Toronto, ON N5H 2LS, Canada. An increase in base strength is indicatedby an increase in pKa value. All of the quoted pKa values werecalculated using the aforementioned ACD software.

As above the heterocyclic compound substituted with electron-releasinggroups may also be substituted on the ring with one or moreelectron-withdrawing groups. Again it is desirable that overall the basestrength is increased compared to the corresponding unsubstitutedcompound.

According to a further aspect, the present invention includes the use ofa composition as defined above for the accelerated hardening of acyanoacrylate adhesive. The composition may be applied to a substratebefore application of the cyanoacrylate adhesive thereto, and/or thecomposition may be applied to the cyanoacrylate adhesive afterapplication of the adhesive to a substrate.

According to a further aspect, the present invention provides anadhesive system comprising a cyanoacrylate adhesive together with acomposition as defined above. Suitably, the composition as defined aboveis held separately from (i.e. does not contact) the adhesive prior toapplication on a substrate.

According to another aspect, the present invention provides a processfor the bonding of substrates or parts, characterised by either of thefollowing series of steps:

-   -   (a) dispensing an activator composition as defined above onto at        least one surface of the substrates or parts to be joined;    -   (b) optionally exposing solvent or other liquid vehicle in the        activator composition to air, optionally with heating and/or        with the aid of a fan;    -   (c) optionally holding the substrate or part having the        activator composition thereon for a retention or shipping        period,    -   (d) applying a cyanoacrylate adhesive to at least one substrate        or part;    -   (e) joining the substrates or parts, optionally with manual or        mechanical fixing,        and    -   (f) optionally subsequently dispensing the activator composition        onto adhesive exposed from a joint gap;        or    -   (i) applying a cyanoacrylate adhesive onto at least one surface        of the substrates or parts to be joined;    -   (ii) joining the substrates or parts, optionally with manual or        mechanical fixing;    -   (iii) dispensing an activator composition as defined above onto        the adhesive before or after the step of joining the substrates        or parts,        and    -   (iv) optionally exposing solvent or other liquid vehicle in the        activator composition to air, optionally with heating and/or        with the aid of a fan.

Suitably the retention or shipping period in step (c) may be in therange from several minutes to several days, for example from two minutesto forty-eight hours. Optionally the activator composition may beapplied onto parts prior to their shipping, forwarding or delivery to anend-user, customer or contractor.

The present invention includes a bonded assembly of substrates or partsbonded by a process as defined above. The present invention alsoincludes as an article of commerce a substrate or part having acomposition as defined above applied thereto.

According to another aspect, the present invention provides an activatorcomposition wherein component (b) the volatile organic solvent is asolvent mixture which comprises a volatile hydrocarbon and a cyclicketone. Cyclic ketones as co-solvents would be expected to achievebetter results in reducing the “halo effect” discussed in co-pendingapplication no. PCT/IE01/00063 than linear ketones such as acetone,butanone, pentanone, hexanone, 4-methyl-2-pentanone, or octanone; thancyclic ethers such as dioxane or tetrahydrofuran; or thanadhesive-miscible solvents such as ethyl acetate.

Suitably, the cyclic ketone is present in an amount of up to about 15%,especially up to about 12%, particularly up to about 10%, by weight ofthe solvent mixture. If an amount substantially greater than 10%, andparticularly greater than 15% is used, there may be a risk that aplastic substrate will be attacked. Desirably, the cyclic ketone ispresent in an amount of at least about 2.5% by weight of the solventmixture. Below this amount the reduction in the “halo effect” may not besufficient for full visual satisfaction. Preferably, the cyclic ketoneis present in an amount of at least about 3% by weight of the solventmixture. At or above this level the presence of cyclic ketone is seen tobe beneficial. Desirably, the cyclic ketone is present in an amount inthe range of 3% to 7.5% by weight of the solvent mixture, particularlyan amount in the range of 4% to 7% by weight of the solvent mixture.

A cyclic ketone may suitably be monocyclic or bicyclic.

Suitably the cyclic ketone is an optionally-substituted cyclic ketone,desirably an alicyclic ketone, having 3-10 carbon atoms in the ring. Asubstituted cyclic ketone maybe mono-or di-substituted on the ring withC₁-C₅ alkyl, more particularly C₁-C₂ alkyl.

One particular example of suitable cyclic ketone is cyclohexanone. Otherexamples include cyclobutanone, cyclopentanone, cycloheptanone and2-methyl cyclopentanone. Examples of bicyclic ketones include2-norbornanone, bicyclo[3.2.1]octan-2-one and bicyclo[2.2.2] octanone.

Desirably, the volatile hydrocarbon is an aliphatic hydrocarbon.Suitably, the volatile aliphatic hydrocarbon may have from 4 to 10carbon atoms, particularly from 5 to 8 carbon atoms, and may be straightchain, branched or cyclic. One particular example of a suitablehydrocarbon is n-heptane.

In one aspect, the present invention relates to use of an activatorcomposition as defined above for the accelerated hardening of acyanoacrylate adhesive, particularly when the activator composition isapplied to the cyanoacrylate adhesive after application of the adhesiveto a substrate.

In an activator composition for the accelerated hardening ofcyanoacrylate adhesives, the activator may suitably comprise a memberselected from the group consisting of:

According to one aspect, the present invention includes the use of anactivator composition as defined above for the accelerated hardening ofa cyanoacrylate adhesive. The composition may be applied to a substratebefore application of the cyanoacrylate adhesive thereto, but moresuitably the composition is applied to the cyanoacrylate adhesive afterapplication of the adhesive to a substrate.

According to a further aspect, the present invention provides anadhesive system comprising a cyanoacrylate adhesive together with anactivator composition as defined above. Suitably, the activatorcomposition as defined above is held separately from the adhesive priorto application on a substrate.

According to another aspect, the present invention provides a processfor the bonding of substrates or parts, characterised by the followingseries of steps:

-   -   (i) applying a cyanoacrylate adhesive onto at least one surface        of the substrates or parts to be joined;    -   (ii) joining the substrates or parts, optionally with manual or        mechanical fixing;    -   (iii) dispensing an activator composition comprising a solution        of one or more activators in a solvent mixture which comprises a        volatile hydrocarbon and a cyclic ketone onto the adhesive        before or after the step of joining the substrates or parts,        and    -   (iv) optionally exposing the solvent mixture in the activator        composition to air, optionally with heating or with the aid of a        fan.

The process of the invention is particularly advantageous when at leastone of the substrates has a surface of a dark colour or is transparentand/or at least one of the substrates is of a plastics material. Howeverthe invention is also useful with substrates of other materials such ascardboard, paper, or wood, particularly if the surface is of a darkcolour.

The present invention includes a bonded assembly of substrates or partsbonded by a process as defined above.

Desirably, an activator composition comprises an amount of activatoreffective to accelerate hardening of a cyanoacrylate adhesive, theactivator being carried in a suitable solvent mixture in accordance withthe invention.

The solutions of the activator(s) may suitably contain the activatorcompound(s) in concentrations of from 0.01 to 10 g per 100 ml of solventmixture; for example, from 0.05 to 5 g of activator substance aredissolved per 100 ml of solvent mixture.

Various conventional organic solvents are suitable as the hydrocarbonsolvent (in the solvent mixture) for the activator(s) according to thisaspect of the present invention, provided they have a sufficiently highvolatility. Desirably, the boiling point of the solvent is below about120° C., suitably below about 100° C., at ambient pressure. Althougharomatic solvents such as toluene or xylene may be used, the hydrocarbonsolvent is desirably an aliphatic hydrocarbon. Suitable solvents includespecialised boiling point gasolines, but especially n-heptane, n-hexane,n-pentane, octane, cyclohexane, cyclopentane, methyl cyclopentane,methyl cyclohexane and isomers of them like isooctane, methylhexanes,methylpentanes, 2,2-dimethyl butane (neohexane), or mixtures thereof, aswell as petroleum benzines and ligroin.

DETAILED DESCRIPTION OF THE INVENTION

An allyl group may be straight-chained, branched or cyclic and may beunsaturated, i.e. the term alkyl as used herein includes alkenyl andalkynyl. The heterocycle may be substituted with one or more alkyl orsubstituted alkyl groups that may be the same or different provided thatat least one of the alkyl or substituted alkyl groups is electronreleasing.

An optionally substituted alkyl group may be substituted with one ormore of the following: halo, CR═CR¹R², CF₃, CCl₃, OC(O)R, COOR, COR, OR,SR, CONR¹R², NO₂, NO₃, SOR, SO₂NR₂, NRSO₂R, SO₂R³, SO₃R³, PO(OR³)₂ andoptionally substituted C₆-C₂₀ aryl, , or aryloxy, CSOR³, COONR³ ₂,NRCOOR, NRCOR, N═N—R^(3, OOR) ³, SSR³, OC(O)OR³, N(OR³)₂, P(OR³)₂, SOR³,OSR³, wherein R, R¹ and R² (which may be the same or different) are H,optionally substituted C₁-C₁₀ alkyl, or optionally substituted C₆-C₂₀aryl, and R³ is optionally substituted C₁-C₁₀ alkyl, or optionallysubstituted C₆-C₂₀ aryl. Halo includes chloro, bromo, fluoro and iodo.Pseudohalo-radicals such as CN, SCN, OCN, NCO, NCS are also includedwithin the group of useful substituents.

With the exceptions mentioned below, some electron-withdrawing groupswhich can be employed for further substitution directly on theheterocycle include OH, OR³, SH, SR³, CR═CR¹R² and aryl, aryloxy orthioaryl.

In the case of substitution of pyridine N oxide in the 4 position thegroups OH, OR³, F, CR═CR¹R² and aryl or aryloxy cause an increase inpKa. Similarly substitution of pyridine in the 4 position with OR³,CR═CR¹R² , aryl or aryloxy cause an increase in pKa. The former andlatter sets of groups may therefore be considered as electron releasing(within the scope of the definition of electron releasing describedpreviously) in the respective cases of 4 substituted pyridine N oxideand 4 substituted pyridine . Substitution of pyridine N oxide in the 3position with hydroxy or alkoxy also causes an increase in the pKacompared to pyridine N oxide

In an organic compound containing the structural element —N═C—S—S—, inwhich the N═C double bond is part of a heterocyclic ring, theheterocyclic ring may be substituted for example with optionallysubstituted C₁-C₁₀ alkyl, optionally substituted C₁-C₁₀ alkoxy,optionally substituted C₁-C₁₀ alkoxyalkyl, halo, CN, CF₃, COOR, COR, OR,SR, CONR¹R², NO₂, SOR, SO₂R³, SO₃R³, PO(OR³)₂ and optionally substitutedC₆-C₂₀ aryl or aryloxy, CSOR³, COONR³ ₂, NRCOOR, N═N—R³, OOR³, SSR³,OOCOR³, NOR³ ₂, ON(COR³)₂, S-aryl, NR³ ₂, SH, OH, SiR³ ₃, Si(OR³)₃,OSiR³ ₃, OSi(OR³)₃, B(OR³)₂, P(OR³)₂, SOR³, OSR³, wherein R,R¹ and R²(which may be the same or different) are H, optionally substitutedC₁-C₁₀ alkyl, or optionally substituted C₆-C₂₀ aryl, and R³ (which maybe the same or different) is optionally substituted C₁-C₁₀ alkyl, oroptionally substituted C₆-C₂₀ aryl.

Desirably, the activator(s) are dissolved in readily volatile organicsolvents, such as hydrocarbons, carboxylic acid esters, ketones, ethersor halogenated hydrocarbons and carbonic acid esters or acetals orketals. The solutions of the activator(s) may suitably contain theactivator compound(s) in concentrations of from 0.01 to 10 g per 100 mlof solvent; for example, from 0.05 to 5 g of activator substance aredissolved per 100 ml of solvent.

When the activator composition contains a mixture of two activatorcompounds, the respective amounts of the activator compounds may varyand are only limited by respective amounts which will no longer beeffective for the desired combination of properties. More particularly,when the activator composition contains a mixture of an aromaticheterocyclic compound substituted with at least one electron-withdrawinggroup and an organic compound having the structural element —N═C—S—S—,the activator compounds may suitably be present in amounts of about 0.1%to about 10% by weight of the said aromatic heterocyclic compound andabout 0.01% to about 5% by weight of the said organic compound, moreparticularly about 0.05% to about 1%, of the said organic compound (c),based on the total weight of the activator composition.

Various conventional organic solvents are suitable as solvents for theactivator(s) according to the present invention, provided they have asufficiently high volatility. Desirably, the boiling point of thesolvent is below about 120° C., suitably below about 100° C., at ambientpressure. Suitable solvents include specialised boiling point gasolines,but especially n-heptane, n-bromopropane, alcohols, for exampleisopropyl alcohol, alkyl esters of lower carboxylic acids, for exampleethyl acetate, isopropyl acetate, butyl acetate, ketones, such asacetone, methyl isobutyl ketone and methyl ethyl ketone. Also suitableare ether solvents, ether esters or cyclic ethers, such as, especially,tetrahydrofuran. In the case of sparingly soluble activators,chlorinated hydrocarbons, such as dichloromethane or trichloromethane(chloroform), may also be used.

The activator compositions according to the present invention aresuitable for the accelerated hardening of conventional cyanoacrylateadhesives which contain as the fundamental constituent one or morecyanoacrylic acid esters, suitably with inhibitors of free-radicalpolymerisation, inhibitors of anionic polymerisation and, optionally,conventional auxiliary substances employed in such adhesive systems,like fluorescence markers.

The activator compositions of the invention may use a combination ofcompounds from within any of the groups (i) to (iii). The particularelectron-releasing group to be employed in any end use application canbe selected for its effect on the activating ability of the activator.In particular the activity of the compound can be “tuned” by appropriatesubstitutions—either on the electron withdrawing group itself or by oneor more additional substituents on the heterocycle. For instanceelectron-withdrawing groups may be employed if a particularelectron-releasing group at a certain position on the heterocycle provestoo activating for an end use. Similarly the position of thesubstitution(s) can be varied as appropriate.

The cyanoacrylic acid esters used in the adhesives are in the main oneor more esters of 2-cyanoacrylic acid. Such esters correspond to thefollowing general formula:H₂C═C(CN)—CO—O—R⁵.

In that formula, R⁵ represents an alkyl, alkenyl, cycloalkyl, aryl,alkoxyalkyl, aralkyl or haloalkyl or other suitable group, especially amethyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, pentyl, hexyl,allyl, methallyl, crotyl, propargyl, cyclohexyl, benzyl, phenyl, cresyl,2-chloroethyl, 3-chloropropyl, 2-chlorobutyl, trifluoroethyl,2-methoxyethyl, 3-methoxybutyl or 2-ethoxyethyl group. Theabove-mentioned cyanoacrylates are known to a person skilled in the artof adhesives, see Ullmann's Encyclopaedia of Industrial Chemistry,Volume A1, p. 240, Verlag Chemie Weinheim (1985) and U.S. Pat. Nos.3,254,111 and 3,654,340. Preferred monomers are the allyl, methoxyethyl,ethoxyethyl, methyl, ethyl, propyl, isopropyl or butyl esters of2-cyanoacrylic acid. The monocyanoacrylic acid esters represent thelargest proportion by weight of the polymerisable monomers in theadhesive.

The mentioned cyanoacrylic acid esters may suitably be present in theadhesives in amounts of from 99.99 to 90 wt. %. Preference is given tocyanoacrylic acid esters the alcohol radical of which is derived fromalcohols having from 1 to 10 carbon atoms, which may also be cyclic,branched or perfluorinated.

The cyanoacrylate adhesives according to the present invention may alsocontain an inhibitor of free-radical polymerisation. Such inhibitorsare, for example, hydroquinone, p-methoxyphenol, but alsosterically-hindered phenols, phenothiazine and the like.

The cyanoacrylate adhesives according to the present invention may alsocontain thickeners as further auxiliary substances. That is desirableespecially when there are to be bonded porous materials which otherwisereadily absorb the low viscosity adhesive. Many types of polymer may beused as thickeners, such as polymethyl methacrylate, other methacrylatecopolymers, acrylic rubber, cellulose derivatives, polyvinyl acetate orpolyalphacyanoacrylate. A usual amount of thickener is generally about10 wt. % or less, based on the total adhesive. In addition to or insteadof the thickeners, the cyanoacrylate adhesives according to the presentinvention may also contain reinforcing agents. Examples of suchreinforcing agents are acrylic elastomers, acrylonitrile copolymers,elastomers or fluoroelastomers. Moreover, inorganic additives may alsobe used, for example silicates, thixotropic agents having a largesurface area, which may be coated with polydialkylsiloxanes.

The cyanoacrylate adhesives according to the present invention may alsocontain substances for increasing the thermal stability thereof. Theremay be used for that purpose, for example, the sulfur compoundsmentioned in European Patent specification No. 579 476.

In addition to or instead of the mentioned additives, the cyanoacrylateadhesives according to the present invention may also containplasticisers. These serve to protect the resulting adhesive bond frombrittleness. Such plasticisers are, for example, C₁-C₁₀ alkyl esters ofdibasic acids, especially of sebacic acid, phthalic acid or malonic acidas well as esters of ethylene glycol, glycol or citric acid. Otherplasticisers are diaryl ethers and polyurethanes and the like.Furthermore, the adhesive preparations according to the presentinvention may also contain colourings, pigments, aromatic substances(fragrances), extenders and the like, as well as fluorescing additives.Reference is directed to U.S. Pat. No. 5,749,956 (Fisher et al.), U.S.Pat. No. 4,869,772 (McDonnell et al.) and U.S. Pat. No. 5,314,562(McDonnell et al.), the contents of which are incorporated herein byreference.

The activator compositions of the present invention are intended to beused with a wide variety of both metallic and non-metallic substrates,including substrates having acidic surfaces such as wood and paper orcardboard, and plastics substrates. A good CTV initiator (activator)should be a sufficiently slow initiator to allow effective initialmixing of the activator through the adhesive prior to polymerisation.

In the aspect of the invention using a cyclic ketone as a co-solvent,the advantage of the activator solutions of the invention isparticularly evident on dark-coloured substrates.

The present invention will now be illustrated in greater detail.

EXAMPLES

In the Examples, the following abbreviations and terms are used:

-   -   DMPT=N,N-dimethyl-p-toluidine    -   s=seconds    -   Heptane=n-heptane    -   CTV=cure through volume    -   ETP=2-ethane thiol pyrazine    -   mins=minutes    -   MPM=6-methyl-2-pyridine-methanol    -   nm=not measured    -   MPPE=2-(3-(6-methyl-2-pyridyl)-propoxy)ethanol.

Loctite 401 (also called 401 herein), is a grade of Loctite ethylcyanoacrylate-based adhesive. It is a low viscosity, fast curing, singlecomponent ethyl cyanoacrylate adhesive (see for example U.S. Pat. No.4,695,615).

Loctite is a trade mark. The above Loctite product is commerciallyavailable from Loctite Corporation, Rocky Hill, Conn., USA or Loctite(Ireland) Limited, Dublin 24, Ireland.

The concentration of activator in an activator solution is expressed as% by weight based on the amount of solvent. The concentration ofcyclohexanone is expressed as % by weight based on the total amount ofsolvent, the remainder being n-heptane.

Post Spray tests were carried out by applying a 10 ul drop of 401adhesive onto a substrate and then spraying a chosen activator onto thedrop. Full Cure Time is the time required for the adhesive drop to curefully.

In pre spray tests the selected activator solution is sprayed onto thesubstrate before application of a 10 ul drop of 401 adhesive. On PartTime is the time interval between application of the spray and additionof the adhesive drop. “Initial” on part time is the time required forthe activator solvent to evaporate leaving the substrate with a dryappearance, this was typically 20-30 seconds.

Pyrazines

Post Spray

The post spray cure speeds of a range of alkyl substituted pyrazines (1%solution in heptane solvent) that contained one or more electronreleasing groups are shown in Table 1. It is evident that many of thepyrazines were more effective CTV agents than DMPT. In the case ofpyrazines that contained electron releasing groups only, substituentsadjacent to both nitrogen atoms showed more optimum reactivity.

Alkyl groups are activating by electron release and thought also to havea counteracting and deactivating effect by steric hindrance of the Natom. Thus, the steric hindrance provided by the tetra methyl derivativecauses it to be the least reactive in terms of skinning speed of thepyrazines tested in Table 1. 2,3 and 2,5-dimethyl pyrazine are lessreactive than 2-methyl pyrazine. The electron releasing effect of themethyl groups renders the aforementioned disubstituted pyrazines morereactive than unsubstituted pyrazine.

2,6-dimethyl pyrazine followed by 2-methyl pyrazine were the two mostreactive pyrazines in terms of skinning speed that were tested. In bothcompounds one of the pyrazine nitrogen atoms is unhindered and theelectron releasing effect of the methyl group(s) increased therespective reactivities compared to non-substituted pyrazine. The rapidskinning of these pyrazines is reflected by their slow CTV times withthe most reactive 2,6-dimethyl derivative having a considerably slowerCTV time (˜14 minutes) than the less reactive 2-methyl derivative (CTV1-3 minutes).

Substitution of an alkyl group in dialkyl substituted pyrazines by anelectron withdrawing and sterically hindering alkoxy or thio groupsreduced the reactivity compared to the dialkyl substituted pyrazine.2-ethane thiol pyrazine (ETP) was less reactive than methyl pyrazine butwas more reactive than unsubstituted pyrazine. TABLE 1 Post spray CTVtimes of pyrazine and Pyrazine derivatives on CTV performance. CTV timeswere measured by spraying a 1% solution of the activator in heptaneusing a 10 ul drop of 401 adhesive on a cardboard substrate. Conc PostSkin Post CTV Pre-CTV Activator (%) (s) (s) (minutes) pKa ± 0.2 DMPT 1.0no skin 14   1-1.25 5.66 Pyrazine 1.0 no skin 10 7 1.0 2,6-Dimethyl 1.0Immediate ˜840 8-10 1.98 2,6-Dimethyl 1.0 Immediate ˜900 8-10 1.982,6-Dimethyl 0.1 5 ˜900 ″ ″ 2,6-Dimethyl 0.01 no skin 600 ″ ″ 2-Methyl1.0 2  60-180 ″ 1.52 2-Methyl 0.1 5 80 ″ ″ 2-Methyl 0.01 no skin 180 ″ ″2-Ethane Thiol 1.0 Immediate 5 8 1.03 ± 0.3 light skin 2,3-Dimethyl 1.02-4 10-15 3 1.97 2,5-Dimethyl 1.0 2-4 10-15 20 1.97 2-methyl 1.0 no skin8-10 2-3 2.06 3-n-propyl 2,3,5- 1.0 no skin 10 3 2.43 Trimethyl Tetramethyl 1.0 no skin 180 4-5 2.88 2-methoxy 1.0 no skin 6 2 1.21 3-methyl2 Methyl 3(5 or 1.0 no skin 7 7 ˜1.27 6) Ethoxy 35 Pyrazine ActivatorPost Skin Post CTV Pre-CTV (1% conc.) (s) (s) (minutes) pKa ± 0.2 DMPTNo skin 14   1-1.25 5.66 Pyrazine no skin 10 7 1.0 2,6-DimethylImmediate ˜840  8-10 1.98 2-Methyl 2  60-180 ″ 1.52 2-Ethane ThiolImmediate 5 8 1.03 ± 0.3  light skin 2,3-Dimethyl 2-4 10-15 3 1.972,5-Dimethyl 2-4 10-15 20 1.97 2-methyl no skin 8-10 2-3 2.06 3-n-propyl2,3,5- no skin 10 3 2.43 Trimethyl Tetra methyl no skin 180 4-5 2.882-methoxy no skin 6 2 1.21 3-methyl 2 Methyl 3(5 or no skin 7 7 ˜1.27 6)Ethoxy 35 Pyrazine

The pre spray cure speed of pyrazine and a range of pyrazines thatcontained at least one electron releasing group is shown in Table 2.Although the initial reactivity of 2-methoxy, 3-methyl pyrazine and thelonger term reactivity of 2-methyl 3-n-propyl pyrazine were similar tothat of DMPT. In all cases the pyrazines had slower CTV times than DMPTover an activator on part times of up to 1 hour. TABLE 2 Pre spray CTVtimes of pyrazine and pyrazine derivatives. Activator Pre-Skin Pre-CTV(1% conc.) On Part Time (s) (minutes) DMPT Initial No skin   1-1.25 15minutes ″ 2.8 1 hour ″ 7 Pyrazine Initial ″ 7 2,3-Dimethyl Initial 20 330 minutes ″ 9 1 hour ″ 12 16 hours ″ >90 2,5-Dimethyl Initial 60 202-methyl Initial 60 2-3 3-n-propyl 30 minutes 7 1 hour 8 16 hours 602,3,5- Initial no skin 3 Trimethyl 2-methoxy Initial ″ 2 3-methyl 20minutes 4 1 hour 20 2 Methyl 3(5 or Initial 7 6) Ethoxy 16 hours 35Pyrazine 2-Ethane Initial  5 8 Thiol tetra methyl Initial  5 4-52,6-Dimethyl Initial ″  8-10

Pyrazines have the important property from a health and safety point ofview that they are relatively non-toxic. Some are actually added tofoods etc. as flavour enhancers. Alkyl, hydroxy, alkoxy and thiopyrazines are for example widely found in food and also added to edibleproducts as flavour

Pyridine N Oxides

Pyridine N oxide was a very slow initiator. Introduction of electronreleasing groups enhanced reactivity. The methoxy group in the 4position (being, in that position, an electron releasing group) appearedto be too activating. However, 4-methyl and 2,4-dimethyl pyridineN-oxide ( i.e. compounds substituted with electron-withdrawing group(s))had good reactivity (CTV 30 seconds). A desirable property exhibited bythe latter accelerator is that it cured the adhesive drop to a cleardrop. The longer term (e.g. 30 minutes) pre spray performance (10 ul401/cardboard substrate) of 4-methyl pyridine N-oxide was superior tothat of DMPT (compare Tables 2 and 3). TABLE 3 Post and Pre sprayreactivity of 1% (unless otherwise indicated) pyridine N-oxidederivatives with 10 ul drop of 401 adhesive on a cardboard substrate.Pyridine-N- Post Spray Pre Spray On Pre Spray Oxide Solvent Conc. (%)(s) Part time (mins) pKa 4-methoxy IPA 1.0 Skin, no CTV — 2.28 4-methoxyIPA 0.1 Skin, no CTV 2.28 2,4-dimethyl IPA 1.0 30 — 1.83 4-methyl IPA1.0 30 initial 4 1.41 4-methyl IPA 1.0 — 30 minutes 4 1.41 4-methyl IPA1.0 — 16 hours 7 1.412,6 Disubstituted Pyridines

Pyridine and 2,6 lutidine were too reactive and only caused skinning inpost spray tests. Replacement of one of the methyl groups in 2,6lutidine with either the slightly electron withdrawing methanol orelectron releasing propoxy ethanol group reduced reactivity sufficientlyto allow good CTV of 401 adhesive. c.f. table 4

TABLE 4 Post spray CTV Performance (401 adhesive) of 2,6 disubstitutedpyridines Activator Post CTV (1%) Solvent (s) pKa 2,6 IPA >300 6.67Lutidine MPM IPA 25-30 5.73 MPPE Heptane 60 6.4pKa Values

The pKa values of the compounds used in the Examples are given in Tables1-4. Those of the parent compounds are given below:

-   -   Activator Calculated pKa (±0.2)    -   Pyridine N Oxide (parent) 0.73    -   Pyridine (parent) 5.32    -   Pyrazine (parent) 1.0    -   DMPT 5.66

Example 1

Post Spray tests were carried out by applying a 10 μl drop of adhesiveonto a substrate and then spraying a chosen activator onto the drop.Full Cure Time is the time required for the adhesive drop to cure fully.

In Pre Spray tests the selected activator solution is sprayed onto thesubstrate before application of a 10 μl drop of adhesive. On Part Timeis the time interval between application of the spray and addition ofthe adhesive drop.

The formulations of the invention show, or would be expected to show, atleast one of the following properties:

-   -   1. No or no substantial loss of surface activation.    -   2. Fast cure after pre-activation.    -   3. No or no substantial shortcomings in cosmetics.    -   4. Fast through cure after post activation.    -   5. No or no substantial loss of bond strengths.

Activator solutions according to this invention would allowmanufacturers to have long waiting periods between the steps ofapplication of activator (surface activation) and application ofadhesive (bonding parts).

Thus the invention can confer the following benefits:

-   -   Interruptions/breaks/hold-ups in production lines do not require        repeated surface activation of the parts to be adhered.    -   Parts to be bonded can be activated in advance by the supplier        or a contractor. This could be advantageous if manufacturer does        not want to equip his production lines with activator        application stages.    -   Large number of parts can be pre-treated in advance and be held        in stock.

Although the invention has been described above, many modifications andequivalents thereof will be clear to those persons of ordinary skill inthe art and are intended to be covered hereby, the true spirit and scopeof the invention being defined by the claims.

1. An accelerator composition for curing of cyanoacrylate adhesives comprising: (a) one or more compounds selected from the group consisting of: (i) pyrazine; or a pyrazine derivative; said pyrazine derivative being pyrazine substituted with at least one electron-releasing group which substituent causes a net increase in the base strength of the substituted compound as compared to the corresponding unsubstituted compound; (ii) pyridine N oxides substituted with at least one electron-releasing group; or (iii) 2,6 pyridines being pyridines substituted in the 2-and 6-positions by substituents, at least one of the substituents being electron-releasing provided that both substituents are not methyl; and (b) a volatile solvent carrier for the compound.
 2. A composition according to claim 1 wherein in a compound of group (iii) at least one of the substituents is more sterically hindering than a methyl group.
 3. A composition according to claim 1 wherein in a compound of group (i) the heterocycle is substituted on at least one of the carbon atoms directly bonded to the N heteroatom.
 4. A composition according to claim 3 wherein the heterocycle is substituted on at least two carbon atoms each carbon atom being directly bonded to one of the N heteroatoms.
 5. A composition according to claim 1 wherein the electron-releasing group(s) are selected from one or more of straight chain, branched chain or cyclic, optionally substituted, alkyl groups.
 6. A composition according to claim 5 wherein said electron-releasing groups are optionally substituted methyl, ethyl, propyl, butyl or pentyl.
 7. A composition according to claim 1 further comprising component (c) an organic compound containing the structural element, —N═C—S—S— or 5-nitro quinoline.
 8. A composition according to claim 1 wherein component (b) the volatile organic solvent is a solvent mixture which comprises a volatile hydrocarbon and a cyclic ketone.
 9. A composition according to claim 8 wherein the volatile hydrocarbon is an aliphatic hydrocarbon.
 10. The reaction product of a composition according to claim 1 with a cyanoacrylate adhesive.
 11. Use a composition according to claim 1 for the accelerated hardening of a cyanoacrylate adhesive.
 12. Use of at least one aromatic heterocyclic compound having at least one N hetero atom in the ring(s) and substituted on the ring(s) with at least one electron-releasing group which group causes a net increase in the base strength of the substituted compound compared to the corresponding unsubstituted compound in the manufacture of a cure through volume accelerator composition for cyanoacrylate adhesives.
 13. Use of at least one compound selected from (i) pyrazines substituted with at least one electron-releasing group which substituent causes a net increase in the base strength of the substituted compound as compared to the corresponding unsubstituted compound; or (ii) optionally substituted pyridine N oxides; or (iii) optionally substituted 2,6 pyridines in the manufacture of a cure through volume accelerator composition for cyanoacrylate adhesives.
 14. An adhesive system comprising a cyanoacrylate adhesive together with a composition as according to claim
 1. 15. A process for the bonding of substrates or parts, characterised by either of the following series of steps: (a) dispensing a composition as according to claim 1 onto at least one surface of the substrates or parts to be joined; (b) optionally exposing solvent or other liquid vehicle in the activator composition to air, optionally with heating and/or with the aid of a fan; (c) optionally holding the substrate or part having the activator composition thereon for a retention or shipping period, (d) applying a cyanoacrylate adhesive to at least one substrate or part; (e) joining the substrates or parts, optionally with manual or mechanical fixing, and (f) optionally subsequently dispensing the activator composition onto adhesive exposed from a joint gap; or (i) applying a cyanoacrylate adhesive onto at least one surface of the substrates or parts to be joined; (ii) joining the substrates or parts, optionally with manual or mechanical fixing; (iii) dispensing a composition as according to claim 1 onto the adhesive before or after the step of joining the substrates or parts, and (iv) optionally exposing solvent or other liquid vehicle in the activator composition to air, optionally with heating and/or with the aid of a fan.
 16. A bonded assembly of substrates or parts bonded by a process according to claim
 15. 17. An article of commerce a substrate or part having a composition according to claim 1 applied thereto.
 18. A process for the bonding of substrates or parts, characterised by the following series of steps: (i) applying a cyanoacrylate adhesive onto at least one surface of the substrates or parts to be joined; (ii) joining the substrates or parts, optionally with manual or mechanical fixing; (iii) dispensing a composition as according to claim 1 wherein component (b) is a solvent mixture which comprises a volatile hydrocarbon and a cyclic ketone, onto the adhesive before or after the step of joining the substrates or parts, and (iv) optionally exposing the solvent mixture in the activator composition to air, optionally with heating or with the aid of a fan.
 19. A bonded assembly formed by the process of claim
 18. 20. A bond formed between two surfaces by the reaction product of claim
 10. 